Embodiments of the subject matter disclosed herein relate to dry gas seals to provide reliable shaft sealing on turbomachinery, particularly centrifugal compressors.
A compressor is a machine which accelerates the particles of a compressible fluid, e.g., a gas, through the use of mechanical energy to, ultimately, increase the pressure of that compressible fluid. Compressors are used in a number of different applications, including operating as an initial stage of a gas turbine engine. Among the various types of compressors are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration which accelerates the gas particles, e.g., by rotating a centrifugal impeller through which the gas is passing. More generally, centrifugal compressors can be said to be part of a class of machinery known as “turbo machines” or “turbo rotating machines”.
Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of impellers in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet conduit for gas to be accelerated, an impeller which is capable of providing kinetic energy to the input gas and a diffuser which converts the kinetic energy of the gas leaving the impeller into pressure energy. Various types of gases are used in centrifugal compressors, some of which are toxic or dangerous to the environment and/or to workers in the plants. Accordingly, centrifugal compressors employ sealing systems, usually placed on the ends of the shaft that supports the impeller(s), to prevent the gas from escaping from the compressor and contaminating the surrounding environment. Single rotor centrifugal compressors are usually provided with two separate seals as part of this sealing system, i.e., one for each end of the shaft, while in a overhung centrifugal compressor it is usually sufficient to seal the shaft end, located immediately downstream of the impeller.
Recently there has been an increase in the use of so-called “dry” gas seals in sealing systems for centrifugal compressors. Dry gas seals can be described as non-contacting, dry-running mechanical face seals which include a mating or rotating ring and a primary or stationary ring. In operation, grooves in the rotating ring generate a fluid-dynamic force causing the stationary ring to separate and create a gap between the two rings. These seals are referred to as “dry” since they do not require lubricating oil which, among other things, greatly reduces their maintenance requirements.
For centrifugal compressors, such dry gas seals are available in different configurations, e.g., so-called tandem configurations which are primarily used in compressors that employ toxic or flammable gases as the input or process gas. As shown in FIG. 1, a tandem-type dry gas seal system includes a first seal 2 and a second seal 4, both contained in a single package. During normal operation of the compressor, the first seal 2 operates to contain the total pressure of gas processed, while the second seal 4 acts as a back-up which is designed to operate only if the first seal 2 fails or leaks excessively. Generally a conditioned gas flow coming from compressor discharge is injected upstream of seal 2 to isolate the dry gas seal from process gas. In the applications with highly toxic process gases (e.g., gas having high contents of H2S) and high sealing pressure, an external sealing gas source having a low sulfur content, e.g., a so-called “sweet” gas is usually provided to isolate the process gas from the surroundings. Due to the high sealing pressure a dedicated reciprocating compressor 6 that operates independently of the centrifugal compressor is used to feed the sealing gas system. The second seal 4 in the tandem may receive a lower pressure (e.g., below 10 Bar) of nitrogen as secondary sealing gas via a source 8 to ensure that no toxic/flammable gas escapes to the surroundings. Centrifugal compressors equipped with these types of dry gas sealing systems thus also require additional compressors whose function is solely to provide the sealing gas, thus making the overall system more complex. In addition to simply adding complexity, reciprocating compressors 6 may have greater maintenance requirements than even the centrifugal compressors which they are intended to serve. Moreover, although the second seal 4 in the tandem configuration does provide a back-up capability, current dry gas seal systems are still not fault free, in which case they may undesirably release a certain amount of sealing gas into the atmosphere.
Dry gas seal failures are mainly due to liquid or solid particles contamination, “hang up” of primary disc causing discs contact, or axial vibration of primary disk (“swashing”). All of them causing severe malfunctioning of the seal.
Accordingly, it would be desirable to design and provide a dry gas seal which overcomes the aforementioned drawbacks of existing sealing systems.